Conventionally, a stratified scavenging two-cycle engine including an air passage that communicates with a scavenging passage has been known (for example, see Patent Document 1). The stratified scavenging two-cycle engine is capable of supplying pure air to an upper portion of the scavenging passage through the air passage, the pure air firstly scavenging combustion gas. As compared with a conventional two-cycle engine in which air-fuel mixture scavenges the combustion gas, the above-described stratified scavenging two-cycle engine is capable of reducing an amount of unburned air-fuel mixture exhausted during scavenging, improving fuel consumption, and cleaning up exhaust gas.
Operation of such a conventional stratified scavenging two-cycle engine during idling will be briefly described below.
FIGS. 20A and 20B are schematic diagrams respectively illustrating an intake process and a scavenging process of the conventional stratified scavenging two-cycle engine during idling.
In the conventional stratified scavenging two-cycle engine during idling, a piston 23 is moved from a bottom dead center to a top dead center, whereby a mixture passage 800 is opened in a crank chamber 25 and a sufficient amount of the air-fuel mixture for idling is delivered into the crank chamber 25 from the mixture passage 800 in the intake process as shown in FIG. 20A. An air valve (not shown) provided in an air passage 700 is generally closed during idling so that the pure air is not delivered from the air passage 700.
When the piston 23 ascends to reach around the top dead center, the air-fuel mixture is ignited to be combusted, i.e. bursted. Due to the burst, the piston 23 starts to descend. When the piston 23 further descends, an exhaust passage (not shown) and a scavenging passage 9 are sequentially opened and the exhaust gas is exhausted from the exhaust passage in the scavenging process as shown in FIG. 20B. At the same time, a part of the air-fuel mixture in the crank chamber 25 is delivered into a cylinder chamber 24 through the scavenging passage 9. Subsequently, the piston 23 starts to ascend from the bottom dead center to repeat the above-described series of procedures.
[Patent Document] JP-A-10-252565